1. Field of the Invention
The present invention relates to a capacitive sensing device in which capacitance is formed between a plurality of driving electrodes and at least one sensing electrode, and more particularly, to a capacitive sensing device of a multi-driving scheme in which a driving signal is simultaneously applied to a plurality of driving electrodes.
2. Description of the Related Art
A mutual capacitive sensing device and a self-capacitive capacitive sensing device are known as capacitive sensing devices including a touch pad. The mutual capacitive sensing device includes a plurality of driving electrodes formed in a column direction, and a plurality of sensing electrodes extending in a row direction. The driving electrode and the sensing electrode are insulated and intersect, and capacitance is formed between the driving electrode and the sensing electrode.
A driving scheme in the mutual capacitive sensing device of the related art is referred to as a 1-hot driving scheme. There is a driving scheme in which a driving signal is sequentially applied to a plurality of driving electrodes, and a change in capacitance at an intersection between the driving electrode and a sensing electrode is obtained from a sensed value from each sensing electrode when the driving signal is applied. However, in the 1-hot driving scheme, since only information on the capacitance at one intersection is obtained from the sensing electrode when the driving signal is applied to the driving electrode, there is a disadvantage in that influence of noise increases and an S/N ratio of capacitance to be detected increases.
On the other hand, in the multi-driving scheme in which a driving signal is simultaneously applied to a plurality of driving electrodes, a sensed value based on capacitance according to the number of a plurality of intersection portions between one sensing electrode and a plurality of driving electrodes is obtained from the one sensing electrode. Therefore, random noise generated when a change in the capacitance in each intersection portion is obtained is averaged and reduced, an S/N ratio is improved, and a relatively high sensitivity is achieved.
In multi-driving of the mutual capacitive sensing device, values of the capacitances at the intersections with the plurality of driving electrodes are detected through superimposition on one sensing electrode. In the driving scheme, a plurality of patterns of a combination of codes in a normal phase (1) and an opposite phase (−1) of the driving signal are prepared, and driving signals based on the respective driving patterns are sequentially applied to the plurality of driving electrodes. When the number of driving electrodes to which the driving signal is simultaneously applied is n, the number of intersections between one sensing electrode and the driving electrodes is n, and accordingly, n driving patterns that are n combinations of the codes of the normal phase (1) or the opposite phase (−1) are used. Thus, a combination of the codes of the driving signal applied to the n driving electrodes is an n×n square matrix.
In the multi-driving scheme, it is possible to obtain decoded values by multiplying the sensed values obtained from the respective sensing electrodes by an inverse matrix of a matrix of the driving pattern. Here, when the code of the decoding matrix is “1”, “1” or “0”, a decoding operation of n sensed values obtained from one sensing electrode can be performed by only addition and subtraction.
Therefore, using a so-called Hadamard matrix as the matrix of the driving pattern has been proposed in Pamphlet of International Publication No. WO2009/107415 below or the like. The Hadamard matrix can include codes “−1” and “1”, an inverse matrix thereof is the same as a transposed matrix of the Hadamard matrix, and a decoding matrix for performing a code operation can include “1” and “−1”. Further, since it is not necessary to set an inverse matrix for a decoding operation different from the matrix of the driving pattern, and the inverse matrix obtained by replacing a row and a column in the matrix of the driving pattern can be used for the decoding operation, the Hadamard matrix is suitable for use for a system with a small memory.
However, in the multi-driving scheme of simultaneously driving a plurality of electrodes, the following problems remain unsolved.
First, when a driving signal is simultaneously applied to a plurality of driving electrodes using a n×n matrix, if a balance of sums of the codes of a normal phase (1) and an opposite phase (−1) of the plurality of driving signals is greatly collapsed, a sensed value obtained from one sensing electrode increases. One example of the Hadamard matrix used as the driving signal is described in paragraph [0065] of Pamphlet of International Publication No. WO2009/107415, but in this matrix, driving signals applied to the plurality of driving electrodes at a timing of a first row are all in a normal phase (1). When all the n driving electrodes are driven using the code “1”, the sensed value becomes n times the sensed value when one driving electrode is driven.
Capacitance (base capacitance) in an intersection portion between the driving electrode and the sensing electrode is very great, whereas a change in capacitance when a finger or the like has approached the intersection portion is very small. In the 1-hot driving scheme of the related art, since the base capacitance in the intersection portion between individual electrodes is substantially constant, a sensing circuit can uniformly cancel the base capacitance, and amplify and read only an amount of the change. On the other hand, in the multi-driving scheme, when the balance of the sum of the codes of respective rows applied to one sensing electrode (at each time) is greatly changed, the base capacitance is not uniformly cancelled, unlike the 1-hot scheme. For example, in a driving scheme using the code shown in the paragraph [0065] of Pamphlet of International Publication No. WO2009/107415, since all eight driving electrodes are driven in phase in the first row, the base capacitance in the row has a value of eight times, a small amount of change in the capacitance placed on the great base capacitance is detected. The balance between a sensed value in this row and sensed values of a second row and subsequent rows is significantly shifted.
When the sensed value is subjected to A/D conversion, an analog value is converted into a digital value with a certain limited resolution. For example, the number of cases of 8 bits is 0 to 255. Here, in the multi-driving scheme, if a large change in balance of the sum of the codes in each row (at each time) is to be detected in a range from 0 to 255 of the same A/D converter, a range of values corresponding to a small amount of change in the capacitance placed on great base capacitance is greatly narrowed in order to convert the analog value to the digital value within the numerical range even when the base capacitance is greatest, and detection resolution is degraded. Thus, when the balance of the sum of the codes in each row (at each time) is greatly changed, it is necessary to increase the bit number of the A/D converter, and a circuit cost increases.
Next, when the balance of the sum of the codes in a normal phase (1) and an opposite phase (−1) of the driving signal simultaneously applied to the plurality of driving electrodes is greatly collapsed, radiated noise emitted from the driving electrode increases with the increase of the sum of codes. In an example of the Hadamard matrix described in paragraph [0065] of Pamphlet of International Publication No. WO2009/107415, since the driving signal of “1” is simultaneously applied to all the driving electrodes at a timing of the first row, the radiated noise greatly increases.
The radiation of the noise can be relatively reduced if a frequency thereof is out of a range of regulation and is, for example, approximately 150 kHz. However, in a typical ASIC, since the driving signal applied to the driving electrode is a rectangular wave, multiplication frequencies are widely included on a high frequency side even when a driving frequency is 150 kHz or lower, and thus, the multiplication frequencies cannot be avoided from a high frequency band. Therefore, when a matrix in which a sum of the codes of the row increases is used, radiated noise increases when the driving electrode is driven at a relatively low frequency.
Then, in paragraph [0071] of Pamphlet of International Publication No. WO2009/107415, the matrix used as the driving signal includes a combination of codes “1”, “4”, and “0”. In this matrix, a maximum sum of the codes of each row is “2”, and the above-described problem of the increase in the sensed value or the radiated noise is solved. However, in this matrix, since the number of driving electrodes to which the driving signal is applied at a timing of each row is limited to 2, effects of averaging of the noise and reduction of the S/N ratio, which are original purposes of the multi-driving scheme, are somewhat degraded.